Do not send zero-datetime (0001-01-01T00:00:00Z) if not set

[iivvoo]

We noticed that voicemessages take very long to send (up to 40sec), while a plain curl invocation will almost call immediately.
It seems to be the way the api deals with the default zero datetime, which will default to 0001-01-01T00:00:00Z.

It lacks the IsZero check that's present in the sms package, for example

[iivvoo]

To clarify a bit further, it seems the backend will always schedule voicemessages with a ScheduledDateTime, even if it's in the past or within seconds in the future, and these tasks are picked up after > 30sec.

The only way to immediately send the voicemessage is to not specify a ScheduledDateTime (or possibly send a null)

Without this fix, sending messages immediately is not possible, which is really annoying for OTP purposes

[iivvoo]

@messagebird thanks for ignoring this PR for nearly a year already. Truely appreciated, very professional

[iivvoo]

@messagebird congrats on ignoring my bug-fixing PR for a whole year! You really know how to make customers and developers happy! 😂

[iivvoo]

@rfeiner I fixed the (by now) broken/outdated test.